FileSystem
{
SteamAppId 240 // This will mount all the GCFs we need (240=CS:S, 220=HL2).
ToolsAppId 211 // Tools will load this (ie: source SDK caches) to get things like materials\debug, materials\editor, etc.

//
// The code that loads this file automatically does a few things here:
//
// 1. For each "Game" search path, it adds a "GameBin" path, in <dir>\bin
// 2. For each "Game" search path, it adds another "Game" path in front of it with _<langage> at the end.
// For example: c:\hl2\cstrike on a french machine would get a c:\hl2\cstrike_french path added to it.
// 3. For the first "Game" search path, it adds a search path called "MOD".
// 4. For the first "Game" search path, it adds a search path called "DEFAULT_WRITE_PATH".
//

//
// Search paths are relative to the base directory, which is where hl2.exe is found.
//
// |gameinfo_path| points at the directory where gameinfo.txt is.
// We always want to mount that directory relative to gameinfo.txt, so
// people can mount stuff in c:\mymod, and the main game resources are in
// someplace like c:\program files\valve\steam\steamapps\<username>\half-life 2.
//
SearchPaths
{
Game |gameinfo_path|.
Game cstrike
Game |all_source_engine_paths|hl2
}
}
}

I'm afraid I'm not sure how this could be happening, could you please remove all plugins other than MM:S and try it, you could then add the MM:S plugins and other VSPs back one at a time to see what breaks.